The present invention relates to Spinal Cord Stimulation (SCS) systems and more particularly to methods for efficiently searching for an effective SCS system stimulation parameter sets. An SCS system treats chronic pain by providing electrical stimulation pulses through the electrodes of an electrode array placed epidurally next to a patient's spinal cord. The stimulation parameter set determines the characteristics of the stimulation pulses provided through the electrode array, and the electrodes used to provide the stimulation pulses, which determines the electric field that is created by the stimulation. The optimal stimulation parameter set for a specific patient may be determined from the response of the patient to various sets of stimulation parameters. There is, however, an extremely large number of possible combinations of stimulation parameters, and evaluating all possible sets is very time consuming, and impractical.
Spinal cord stimulation is a well accepted clinical method for reducing pain in certain populations of patients. An SCS system typically includes an Implantable Pulse Generator (IPG), electrodes, electrode lead, and electrode lead extension. The electrodes are implanted along the dura of the spinal cord, and the IPG generates electrical pulses that are delivered, through the electrodes, to the dorsal column and dorsal root fibers within the spinal column. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing in order to create an electrode array. Individual wires within one or more electrode leads connect with each electrode in the array. The electrode leads exit the spinal column and generally attach to one or more electrode lead extensions. The electrode lead extensions, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG is implanted.
Spinal cord stimulators and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940 issued Mar. 7, 1972 for “Implantable Electronic Stimulator Electrode and Method” that provides timed sequenced electrical impulses to a plurality of electrodes. As another example, U.S. Pat. No. 3,724,467 issued Apr. 3, 1973 for “Electrode Implant For The Neuro-Stimulation of the Spinal Cord,” teaches an electrode implant for the neuro-stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by leads to an RF receiver, which is also implanted.
In U.S. Pat. No. 3,822,708, issued Jul. 9, 1974 for “Electrical Spinal Cord Stimulating Device and Method for Management of Pain,” another type of electrical spinal cord stimulation device is taught. The device disclosed in the '708 patent has five aligned electrodes which are positioned longitudinally on the spinal cord. Electrical pulses applied to the electrodes block perceived intractable pain, while allowing passage of other sensations. A patient operated switch allows the patient to adjust the stimulation parameters.
Most of the electrode arrays used with known SCS systems employ between 4 and 16 electrodes. Electrodes are selectively programmed to act as anodes, cathodes, or left off, creating a stimulating group. The number of stimulation groups available, combined with the ability of integrated circuits to generate a variety of complex stimulation pulses, presents a huge selection of stimulation parameter sets to the clinician. When an SCS system is implanted, a “fitting” procedure is performed to select an effective stimulation parameter set for a particular patient.
A known practice is to manually test one parameter set, and then select a new stimulation parameter set to test, and compare the results. Each parameter set is painstakingly configured, and ramped up in amplitude gradually to avoid patient discomfort. The clinician bases their selection of a new stimulation parameter set on their personal experience and intuition. There is no systematic method to guide the clinician. If the selected stimulation parameters are not an improvement, the clinician repeats these steps, using a new stimulation parameter set, based only on dead-reckoning. The combination of the time required to test each parameter set, and the number of parameter sets tested, results in a very time consuming process.
An example of another stimulation system that is known in the art is a cochlear implant, such as the implant and system described in U.S. Pat. No. 5,626,629, issued May 6, 1997, entitled “Programming of a Speech Processor for an Implantable Cochlear Stimulator” and incorporated herein by reference. The '629 patent describes a method for fitting a cochlear implant to a patient. The method involves determining estimated and threshold stimulation levels of one of the channels of the implant using an objective measurement, such as a measured electrically evoked physiological response. This information is used as a starting point to make further adjustments to stimulation parameters in response to subjective feedback from the patient.
Another known practice is current steering, a process that is more fully described in U.S. Pat. No. 6,393,325, incorporated herein by reference. This process greatly reduces the amount of time required to test a parameter set because the stimulation moves gradually along the array and does not need to be ramped down and then up again in between the testing of different parameter sets as in a conventional system. For example, one embodiment disclosed in the U.S. Pat. No. 6,393,325 (noted above) uses a table having stimulation parameters and a directional input device which the patient uses to navigate through the table.
What is needed is a method for selection of trial stimulation parameter sets that guides the clinician towards an effective stimulation parameter set(s). What is also needed is an algorithm to maintain constant paresthesia while stimulation is transitioned from one electrode to another.